The supply of a nonrenewable resource such as oil, coal and natural gas are essentially fixed (for all practical purposes since they replenish themselves at a negligible rate in relation to rates of human extraction). Yet as demand has increased, newly discovered supplies coupled with more effective extraction technologies have resulted in expanded reserves (accessible global stocks), and at lower costs. Predictions of a turning point in the availability of fossil fuels vary widely, and there is great uncertainty associated with future discoveries and improved technologies for extraction. At the same time that exploration and extraction continues, concerns are voiced about the expense of deeper drilling and more costly extraction and transportation. Increasingly there is a view that, although fossil fuels are available in very large supplies (especially coal), the limiting factor on continued reliance on fossil fuels may be environmental limits due to the climate change impacts.